There have been various single valve controls for use in either an automotive type control or an anti-stall control for a prime mover driven hydrostatic transmission. The term automotive type control, when used relative to hydrostatic transmissions, refers to a displacement control which is responsive to pump input speed, and thus driving prime mover speed, to gradually increase the transmission ratio as prime mover speed increases above a predetermined rpm such as engine idle. In one previous automotive control taught in a paper numbered 851505 entitled, "An Automotive Type Hydrostatic Transmission Control", by Peter J. Strommel and Charles Sticklin, a single speed sensing valve, also referred to as a type DA Automotive Style Cartridge Valve, utilizes the .DELTA.P generated across a fixed orifice to position a valve spool in response to flow from an input speed pump. This valve also has output feedback applied against a lesser area to provide a multiplication gain in output relative to speed pump flow generated .DELTA.P. The output of the speed sensing valve is used to operate a swashplate servo mechanism in response to engine speed. The particular construction of the cartridge valve taught in this 851505 article makes it impossible, or at least difficult, to manufacture and assembly the valve. Furthermore, since the flow is generated across a fixed orifice, the generated .DELTA.P is not linear, and thus does not provide a linear output.
Another example of an automotive control is taught in U.S. Pat. No. 3,986,358 issued to Helmuth Koffmann on Oct. 19, 1976 wherein the displacement control flow to both the variable displacement pump and a variable displacement motor in a transmission are controlled by generated input speed pump flow across a manually controlled (gas pedal) variable orifice. As the .DELTA.P across the variable orifice increases, a bleed valve from the control line is gradually closed to increase the pressure to the servo mechanisms. This particular control has no feedback, generates no gain in the power control, and furthermore does not provide a linear control relationship between speed pump flow and hydraulic unit displacement since there is no pressure control of the size of the variable orifice.
It is also known in anti-stall controls to use a fixed displacement pump to provide a hydraulic speed signal which is sensed across an orifice to generate a .DELTA.P to destroke the hydrostatic transmission pump to reduce prime mover load. This may be done by applying the anti-stall signal to move the displacement control valve as taught in U.S. Pat. No. 3,946,560 issued to McIntosh et al on Mar. 30, 1976. U.S. Pat. No. 3,803,843, issued to Wyman et al, on Apr. 16, 1974, teaches using a fly weight valve to generate a speed signal which is applied against an accelerator adjusted spring force to position a govenor valve to modulate servo control pressure and also is applied against a spring and feedback biased pressure regulator valve which acts as a gain amplifier and provides charge supply to the govenor valve. In various control stages, this control provides both automotive type and anti-stall functions, but requires three valves and the flow generated .DELTA.P does not provide a linear control curve.